This invention relates to ultrasound imaging systems and, more particularly, to increasing the harmonic-to-fundamental ratio and the harmonic-to-noise ratio of tissue-generated and contrast generated harmonic signals in medical ultrasound imaging.
Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements for transmitting an ultrasound beam and receiving the reflected beam from the object being studied. Such scanning comprises a series of measurements in which the focused ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received, beamformed and processed for display. Typically, transmission and reception are focused in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
For ultrasound imaging, the array typically has a multiplicity of transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. The beamforming parameters of each of the firings may be varied to provide a change in focus or direction for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. For a steered array, by changing the time delays and amplitudes of the applied voltages, the beam with its focal point can be moved in a plane to scan the object. For a linear array, a focused beam directed normal to the array is scanned across the object by translating the aperture across the array from one firing to the next.
The same principles apply when the transducer probe is employed in a receive mode to receive the reflected sound. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound energy reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delays (and/or phase shifts) and gains to the signal from each receiving transducer element.
An ultrasound image is composed of multiple image scan lines. A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest, and then receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. After the transmission, one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or time delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. A scan line resolution is a result of the directivity of the associated transmit and receive beam pair.
The output of the beamformer is detected to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed, scan-converted and then displayed as an image of the anatomy being scanned.
Conventional ultrasound transducers transmit a broadband signal centered at a fundamental frequency f0, which is applied separately to each transducer element of the transmit aperture by a respective pulser. The pulsers are activated with time delays that produce the desired focusing of the transmit beam at a particular transmit focal position.
As the transmit beam propagates through tissue, echoes are created when the ultrasound wave is scattered or reflected from the boundaries between regions of different density. The transducer array transduces these ultrasound echoes into electrical signals, which are processed to produce an image of the tissue. These ultrasound images are formed from a combination of fundamental (linear) and harmonic (nonlinear) signal components, the latter of which are generated in nonlinear media such as tissue or a blood stream containing contrast agents. With scattering of linear signals, the received signal is a time-shifted, amplitude-scaled version of the transmitted signal. This is not true however for acoustic media which scatter nonlinear ultrasound waves.
The echoes from a high-amplitude signal transmission will contain both linear and nonlinear signal components. In some instances ultrasound images may be improved by suppressing the fundamental and emphasizing the harmonic (nonlinear) signal components. If the transmitted center frequency is f0, then tissue/contrast nonlinearities will generate harmonics at Nf0 and subharmonics at f0/N, where N is an integer greater than or equal to 2. [The term xe2x80x9c(sub)harmonicxe2x80x9d refers to harmonic and/or subharmonic signal components.] Imaging of harmonic signals has been performed by transmitting a narrow-band signal at frequency f0 and receiving at a band centered at frequency 2f0 (second harmonic) followed by receive signal processing.
Tissue-generated harmonic imaging is capable of greatly improving B-mode image quality in difficult-to-image patients. One problem faced by tissue-generated harmonic imaging is low harmonic-to-noise ratio (HNR) since the harmonic signals are at least an order of magnitude lower in amplitude than the fundamental signal. A secondary problem is insufficient isolation of the harmonic signal from the fundamental as measured by a low harmonic-to-fundamental ratio (HFR).
Coded excitation is a well-known technique in medical ultrasound imaging. For example, the use of Barker codes is disclosed in commonly assigned U.S. Pat. No. 5,938,611, issued Aug. 17, 1999 and the use of Golay codes is disclosed in commonly assigned U.S. Pat. No. 5,984,869, issued Nov. 16, 1999.
The techniques of tissue harmonic imaging and harmonic imaging using contrast agents are likewise known. The technique of tissue harmonic imaging is presented in Averkiou et al., xe2x80x9cA New Imaging Technique Based on the Nonlinear Properties of Tissues,xe2x80x9d 1997 IEEE Ultrasonics Symp., pp. 1561-1566, while harmonic imaging using contrast agents is presented in de Jong et al., xe2x80x9cPrinciples and Recent Developments in Ultrasound Contrast Agents,xe2x80x9d Ultrasonics, Vol. 29, 1991, pp. 324-330, and in Uhlendorf, xe2x80x9cPhysics of Ultrasound Contrast Imaging: Scattering in the Linear Range,xe2x80x9d IEEE Trans. Ultrason. Ferroelec. and Freq. Control, Vol. 41, No. 1, pp. 70-79, January (1994). Tissue harmonics can greatly improve B-mode image quality in difficult-to-image patients, while contrast harmonics can greatly improve vascular studies.
The technique of phase shifting the transmit signal across the transmit aperture to cancel out the transmitted signal at the second harmonic frequency is disclosed by Krishnan et al. in xe2x80x9cTransmit Aperture Processing for Nonlinear Contrast Agent Imaging,xe2x80x9d Ultrasonic Imaging, Vol. 18, pp. 77-105, 1996.
Takeuchi has extended the phase shifting concept to coded excitation of contrast-generated second harmonic signals in xe2x80x9cCoded Excitation for Harmonic Imaging,xe2x80x9d 1997 IEEE Ultrasonics Symp., pp. 1433-1436.
In medical ultrasound imaging systems of the type described hereinabove, it is desirable to optimize the HFR and HNR. In particular, there is a need for a system and a method for significantly increasing the HFR and HNR in harmonic imaging.
In a preferred embodiment of the invention, performance of tissue-generated harmonic imaging using coded excitation improves the HFR and HNR of tissue-generated harmonic signals by transmitting a long encoded pulse sequence and decoding the received beamsummed data.
The transmitted pulse sequence amplitude is set sufficiently high to generate harmonic signals from the tissue nonlinearity. The harmonic signals are received (along with the fundamental signal), beamformed, isolated and decoded, and used to form an image.
In a preferred embodiment of the invention, the transmit waveform for acquiring the N-th harmonic signal is biphase (1,xe2x88x921) encoded using two code symbols of a code sequence, each encoded portion (i.e., chip) of the transmit waveform encoded with the second code symbol being phase-shifted by 180xc2x0/N relative to the chips encoded with the first code symbol. This is implemented by time shifting the chips of the transmit sequence encoded with the second code symbol by xc2xdN fractional cycle at center frequency relative to the chips encoded with the first code symbol. For the second harmonic signal (N=2), the phases of the two chips of the encoded transmit sequence are 90xc2x0 apart, which is implemented by circularly shifting the second chip by a quarter cycle in the transmit sequence memory. [The term xe2x80x9ccircularly shiftingxe2x80x9d as used herein means that the time samples which are dropped at the front end of a shifted chip are added at the back end of the shifted chip.] During reception, the second harmonic signal is isolated by a bandpass filter centered at twice the fundamental frequency and enhanced with decoding. The bandpass filtering and decoding functions are preferably combined in one filter.
Increased HFR is realized since only the second harmonic signal is properly matched to the decoding filter while the fundamental (and other harmonics) are not properly encoded and do not achieve any decoding gain. Using this technique, both the HFR and HNR of the second harmonic signal increase by 10log(n) dB, where n is the number of chips in the single-transmit (e.g., Barker) code or by 10log(2n) for a two-transmit (e.g., Golay) code. The additional HFR gain allows broader-band signals to be used for improved resolution.